A New Technique to Study the Interfacial Strength and Transverse Cracking Scenario in Composite Materials

A new technique has been developed to study interfacial debonding and subsequent coalescence into transverse cracks using a loading stage in an environmental scanning electron microscope. This technique has been used to study the effect of moisture on the interfacial strength and transverse cracking behavior of unaged composite specimens (≈ 1.1% of matrix weight is moisture) compared to composite specimens saturated by soaking in seawater (≈ 2.2% of matrix weight is moisture). Specimens experienced both initiation controlled failure and propagation controlled failure. The type of failure did not appear to depend upon the moisture content; rather it depended upon the microstructure of the composite. Analysis has also been performed, aiding in the interpretation of the experimental results. The mathematical models show that the interfacial residual stresses in the radial direction due to thermal cooldown are compressive in a homogeneous microstructure; while in an inhomogeneous microstructure, the stresses are tensile at the boundary of the inhomogeneity. In both cases the presence of moisture relaxes the stresses.